1. Field of the Invention
The present invention pertains to a primer composition and the preparation and use thereof. More particularly, the invention pertains to a primer composition for silicone elastomers (“rubbers”), in particular for addition-crosslinking silicone rubbers, the preparation thereof, and to a process for improving the adhesion of silicone rubbers to substrates, in particular to metal substrates.
2. Background Art
Silicone elastomers have properties which make them preferable to other elastomers in many applications, an example being their thermal stability over a wide temperature range. In some applications strong bonds between a soft elastomeric material such as a silicone rubber, and a hard substrate, for example, a metal or a thermoplastic, are required. If an adequate bond to the substrate cannot be achieved by self-adhesive rubbers by means of inherent characteristics or through use of the suitable additives, the bond strength can be improved by pretreatment of the substrate surface with a suitable primer.
The efficiency of the primer is dependent both on the composition and surface characteristics of the substrate, and on the composition, the crosslinking system, the additives, and the viscosity of the silicone rubber which is to be adhered. According to the prior art, primer coats which are combinations of tetraalkoxysilanes, or partial hydrolysis products of tetraalkoxysilanes with metal alkoxides or metal chelates are used. These are optionally used in combination with a suitable solvent.
U.S. Pat. No. 2,751,314 describes a method for increasing adherence of silicone rubber to solid substrates, in which a coating consisting of a titanium alkoxide and an alkyl polysilicate or partial hydrolysis product thereof is applied to the substrate.
European published Application EP 0 347 049 A2 discloses improved primer compositions for the adhesion of silicone elastomers to inorganic substrates, such as glass or metal. The primer coat contains a tetraalkyl titanate, at least one alkyl orthosilicate and a hydrocarbon solvent, the solvent having a composition such that from 2 to 25% by weight of the total composition consists of an alcohol having from 4 to 20 carbon atoms. The alcohol present as solvent, preferably n-butanol, maintains the adhesion produced by the primer coat even after storage of the bond in water.
Published EP 0 241 851 A2 describes a method for coating substrates with a primer coat containing a tetraalkyl titanate, an organyloxysilane, for example tetraethyl orthosilicate, and an organic solvent. The primer coat serves here as a protective coating.
U.S. Pat. No. 4,356,233 discloses primer compositions containing a silane which contains no amino or amido functionality, a metal ester, preferably an inorganic acid, and an organic solvent. A preferred silane is methacryloyloxypropyltrimethoxysilane. The primer coat is cured by heat after application to the substrate before a further polymerizable composition containing epoxy- or vinyl-containing monomers is applied.
European patent EP 0 781 823 B1 discloses primer compositions consisting of an alkenyl trialkoxysilane, a platinum compound, by means of which the binding of the rubber to the primer coat is improved, an organic titanate ester and a solvent. The alkenyl group of the alkenyl trialkoxysilane contains minimally three carbon atoms.
US published application 2003/0116273 A1 discloses a method for adhesively bonding optical components such as glass lenses or optical fibers to an adhering resin compositions by means of a primer composition. The primer composition contains either a trialkoxysilane carrying epoxy groups or a trialkoxysilane carrying alkenyl groups, and furthermore contains a tetraalkoxysilane, a metal alkoxide and a solvent. Such compositions lead to excellent heat stability and excellent parting forces and suppress the formation of bubbles during cure, thus preventing reduced transparency in the optical components.
International Patent Application WO 2001/88049 A1 describes primer compositions containing at least one tetraalkoxysilane and/or at least one alkyl polysilicate, at least one metal chelate and/or metal alkoxide, and at least one organic solvent. The tetraalkoxysilane and/or alkyl polysilicate have a specific degree of condensation, expressed as a ratio of tetraalkoxysilane Si(OR)4 to silyloxy units condensed to different degrees. The preferred ratios in the composition are 0≦Si(OR)4≦5, 0≦SiO1/2(OR)3≦30, 0≦SiO2/2(OR)2≦75, 20≦SiO3/2(OR)≦100 and 0≦SiO4/2≦25. The primer composition according to WO 2001/88049 A1 is used for bonding addition-crosslinking RTV-2 silicone rubber to aluminum or epoxy resin substrates. As a result of the use of more highly condensed tetraalkoxysilanes in the primer coats, the composition is less sensitive to moisture and leads to cohesive tearing in the rubber, even after storage of the composites in water.
The prior art therefore describes a number of primer compositions and methods for bonding silicone rubber to substrates. However, the known primer compositions have proven to be inadequate under certain conditions, such as, for example, dynamic loading of the composites.